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. 2024 Jul 3;146(28):19350–19359. doi: 10.1021/jacs.4c05394

A Sterically Accessible Monomeric Stibine Oxide Activates Organotetrel(IV) Halides, Including C–F and Si–F Bonds

John S Wenger 1, Timothy C Johnstone 1,*
PMCID: PMC11258792  PMID: 38959432

Abstract

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Phosphine oxides and arsine oxides are common laboratory reagents with diverse applications that stem from the chemistry exhibited by these monomeric species. Stibine oxides are, in contrast, generally dimeric or oligomeric species because of the reactivity-quenching self-association of the highly polarized stiboryl (Sb=O/Sb+–O–) group. We recently isolated Dipp3SbO (Dipp = 2,6-diisopropylphenyl), the first example of a kinetically stabilized monomeric stibine oxide, which exists as a bench-stable solid and bears an unperturbed stiboryl group. Herein, we report the isolation of Mes3SbO (Mes = mesityl), in which the less bulky substituents maintain the monomeric nature of the compound but unlock access to a wider range of reactivity at the unperturbed stiboryl group relative to Dipp3SbO. Mes3SbO was found to be a potent Lewis base in the formation of adducts with the main-group Lewis acids PbMe3Cl and SnMe3Cl. The accessible Lewis acidity at the Sb atom results in a change in the reactivity with GeMe3Cl, SiMe3Cl, and CPh3Cl. With these species, Mes3SbO formally adds the E–Cl (E = Ge, Si, C) bond across the unsaturated stiboryl group to form a 5-coordinate stiborane. The biphilicity of Mes3SbO is sufficiently potent to activate even the C–F and Si–F bonds of C(p-MeOPh)3F and SiEt3F, respectively. These results mark a significant contribution to an increasingly rich literature on the reactivity of polar, unsaturated main-group motifs. Furthermore, these results highlight the utility of a kinetic stabilization approach to access unusual bonding motifs with unquenched reactivity that can be leveraged for small-molecule activation.

Introduction

There is growing interest in the design of main-group systems that can engage in the types of reactivity that are typically mediated by transition metals to provide sustainable alternatives but use cheap, Earth-abundant elements.1,2 Transition metal complexes are often employed to activate strong σ bonds by using their ability to cycle between oxidation states and coordination numbers to oxidatively add and reductively eliminate substrates. Redox-active pnictogen compounds have recently been identified as particularly suitable to mediate similar reaction pathways with main-group elements.3,4 For example, the activation of C(sp2)–F bonds in catalytic hydrodefluorination has now been achieved using an N,C,N-pincer complex of Bi(I),5 geometrically constrained phosphorus species,6,7 and even simple phosphines.8

Beyond metallomimetic chemistry, the unique properties of p-block elements can be exploited to unlock reaction pathways and synthetic strategies that are entirely unprecedented or serve a complementary role to d-block elements. A notable example is the activation of C(sp3)–F bonds, which can be achieved by potent main-group Lewis acids such as silylium cations.9,10 The related Si–F bonds are among the strongest covalent single bonds, and their functionalization often poses an even more formidable challenge, as reflected in the bond dissociation energies of H3C–F (D°298 = 460.2 ± 8.4 kJ mol–1) and H3Si–F (D°298 = 638 ± 5 kJ mol–1).11 Because of their stability, the formation of fluorosilanes such as SiEt3F often serves as the thermodynamic driving force for catalytic defluorination reactions. The activation of Si–F bonds by transition metals is rare, and examples are limited to fluorosilanes with pendent phosphine groups that direct the active metal center to engage in the bond activation.12,13 Recently, neutral Lewis superacids14 competent in the abstraction of fluoride from SiEt3F have been developed and show promise in overcoming issues with solubility and functional group tolerance associated with silylium salts.15,16

There have also been extensive efforts toward the development of organopnictogen(V) Lewis acids that can engage in C–F bond activation.17 Sb-containing species have a particularly strong ability to interact with fluorine and have been developed for applications in ion-sensing, ion-transport, and catalysis.18−26 Notably, the air-stable salt [Sb(C6F5)4][B(C6F5)4] demonstrated Lewis superacidity in the abstraction of fluoride from [SbF6]− and [BF(C6F5)3]− and promoted the catalytic hydrodefluorination of fluoroalkanes in the presence of THF through the generation of Et3Si+ from Et3SiH.27

The stabilization of unsaturated main-group compounds by sterically demanding substituents has now afforded many compounds that exhibit inherent unquenched reactivity that can be leveraged for small molecule activation.28,29 Unsaturated bonds between two different elements exhibit a polarity that alters the reactivity of the species. Of particular note are recent advances in the isolation of carbonyl analogues of the general form R2E=O–/0/+ (E = B, Si, Ge, P).30 If the unsaturated E–O bond is highly polarized, then it may favor the addition of relatively inert, polar covalent substrates.

Our group has been interested in molecules that feature Sb–O bonds with relevance in medicinal and synthetic organometallic chemistry.31−34 Unlike phosphine oxides and arsine oxides of the form R3Pn=O/R3Pn+–O– (Pn = P, As), which readily exist as tetrahedral monomers, stibine oxides are typically dimeric or oligomeric with Sb–O single bonds.35−38 The currently accepted model of pnictoryl bonding (Pn=O/Pn+–O–) involves a polar covalent single bond stabilized by backdonation from O-centered lone pairs to Pn–C σ* orbitals.39 The remarkable stability of the phosphoryl bond can be exploited to drive reactions to completion,40−43 and its polarity allows phosphine oxides to find diverse applications in coordination chemistry,44 uranium capture,45 supramolecular synthesis,46 medicine,47 and catalysis.48,49

For heavier Pn atoms, increased size and diffuseness of valence orbitals reduce overlap with O-centered lone pairs, disrupting π-backdonation and resulting in a more polarized pnictoryl bond.50 In the case of stibine oxides, the enhanced polarity of the stiboryl bond coupled with a greater propensity for the larger Sb atom to expand its coordination sphere results in self-association of monomers to form multimeric species in the absence of other stabilizing interactions.51−53

As part of our ongoing study of unsaturated Sb–O bonds,54,55 we recently synthesized Dipp3SbO (Dipp = 2,6-diisopropylphenyl), the first isolated monomeric stibine oxide featuring an unperturbed stiboryl group (Sb=O/Sb+–O–).56 The Dipp substituents are sufficiently sterically demanding to kinetically stabilize a monomeric, bench-stable species that can engage in H-bonding, Bro̷nsted base chemistry, coordination chemistry, and O atom transfer. The Bro̷nsted basicity of Dipp3SbO is dramatically enhanced over that of the lighter congeners,57 and our molecular orbital calculations suggested that Dipp3SbO enjoys a similarly privileged Lewis acidity.56 Access to the Lewis acidic Sb atom is, however, largely precluded by the steric shielding of the Dipp substituents.

Herein, we report the isolation of a novel monomeric stibine oxide, Mes3SbO. The use of less sterically demanding Mes substituents enables Mes3SbO to engage in a more extended range of reactions than Dipp3SbO. A theoretical investigation of Mes3SbO was conducted, and we found a negligible degree of variation in the electronic structure of the stiboryl bond relative to that of Dipp3SbO, suggesting that differences in reactivity between these two species are likely to arise primarily from steric factors. We describe here that Mes3SbO forms stable Lewis adducts with the main-group Lewis acids PbMe3Cl and SnMe3Cl. In contrast, treatment of Mes3SbO with GeMe3Cl, SiMe3Cl, or CPh3Cl results in addition products of the general form, Mes3Sb(OER3)Cl (ER3 = GeMe3, SiMe3, CPh3). The ability of Mes3SbO to access the biphilicity of the stiboryl group in addition chemistry prompted us to pursue more challenging C–F and Si–F bond activations. Treatment of Mes3SbO with C(p-MeOPh)3F results in the formation of the expected C–F activation product, Mes3Sb(OC(p-MeOPh)3)F. Most remarkably, treatment of Mes3SbO with SiEt3F results in the clean formation of Mes3Sb(OSiEt3)F within minutes at room temperature. More broadly, the activation of C(sp3)–F and Si–F bonds by the stiboryl group highlights the ability of polar, unsaturated main-group motifs to achieve such challenging bond activations.

Results

Synthesis and Characterization of Mes3SbO

Phosphine oxides and arsine oxides are commonly used as Lewis bases, and in our earlier work we studied the interaction between Dipp3SbO and transition-metal centers.56 This coordination chemistry, in combination with the dramatically enhanced Bro̷nsted basicity of Dipp3SbO relative to its lighter congeners,57 led us to anticipate that the stibine oxide would readily form stable Lewis adducts with a wide variety of main-group Lewis acids. As an initial test of this capability, we investigated its reaction with PbMe3Cl, which features a sterically unencumbered Pb atom capable of expanding its coordination sphere and a low-lying Pb–Cl σ* orbital into which it can accept electron donation. Despite these favorable characteristics, 1H NMR analysis of a CDCl3 solution of Dipp3SbO and excess PbMe3Cl revealed no significant interactions between the two (Figure S1). We suspect that this lack of reactivity results from the steric shielding of the bulky Dipp substituents. Although the Dipp substituents prevent self-association and hydrolysis, this stability comes at the cost of attenuated reactivity with substrates that could otherwise engage with the stiboryl group. Decreasing the bulk of pnictine substituents can indeed have a drastic impact on their reactivity, as we recently demonstrated by comparing the oxidative halogenation of Mes3Pn and Dipp3Pn (Pn = Sb, Bi).58

Prior studies with Mes-substituted Sb compounds suggested that a Mes3Sb framework, although more open, is still sufficiently encumbered to prevent deactivating self-association upon oxidation. Specifically, treatment of Mes3Sb with H2O2 yields Mes3Sb(OH)2, the monomeric structure of which was confirmed with X-ray crystallography and EXAFS.56,59 Decreasing the steric bulk further to simple Ph groups results in polymerization upon oxidation, although we note that monomeric Ph3Sb(OH)2 has been isolated via the oxidative hydrolysis of [Et4N][PhSbCl2Br] in methanol.35−38,60 We interpret a prior report that treatment of Mes3Sb with iodosobenzene affords Mes3Sb(OH)2 as confirmation that the Mes3Sb framework is not only capable of supporting a monomeric stibine oxide, but that the resulting stiboryl unit is sufficiently accessible that it rapidly hydrolyzes in the presence of adventitious water.61 Such facile hydrolytic behavior of the putative Mes3SbO stands in stark contrast to the relative stability of Dipp3SbO, which can be heated in the presence of excess water with no effect. Thus, we sought to isolate Mes3SbO to more deeply probe the reactivity of the stiboryl group.

Under rigorously dry conditions, treatment of Mes3Sb with a suspension of PhIO in DCM resulted in the rapid consumption of the solid oxidant to produce Mes3SbO (Scheme 1), which could be isolated as a colorless crystalline solid on a multigram scale. The 1H and 13C NMR spectra of the product reveal a single mesityl environment averaged by a rapid rotation about the Sb–Cipso bond. No decoalescence or significant broadening of the ortho-methyl or meta-proton signals was detected by low-temperature 1H NMR experiments (Figure S6). In contrast, the room-temperature 1H and 13C NMR spectra of Dipp3SbO display a single Dipp environment that is desymmetrized by restricted rotation about the Sb–Cipso axis; the difference in the magnetic environments of the O-proximal isopropyl groups and the Sb-proximal isopropyl groups is accentuated by serendipitous intramolecular H-bonding with the stiboryl O atom.56 These observations provide preliminary insight into the difference in the steric environment about the stiboryl groups of the two compounds. In Mes3SbO, the ortho-methyl protons of the mesityl groups are deshielded as compared to Mes3Sb, consistent with other oxidation products of this stibine. The IR spectrum of Mes3SbO is in excellent agreement with the calculated (PBE0/def2-TZVPP) spectrum for monomeric Mes3SbO (Figure S4). The correspondence of the Mes3SbO stiboryl stretching frequency, ν(Sb–O) = 778 cm–1, with that of Dipp3SbO (779 cm–1) suggests that despite the significant change in sterics between the two species, there is little variation in the electronic structure of the stiboryl group.

Scheme 1. Synthesis of Mes3SbO.

Scheme 1

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The solid-state structure of Mes3SbO was determined by single crystal X-ray diffraction (SC-XRD) (Figure 1). Mes3SbO crystallizes as a tetrahedral monomer in space group P21/c. The stiboryl Sb–O bond length is 1.848(2) Å, in agreement with that of Dipp3SbO (1.8371(4) Å), and significantly shorter than the Sb–O bond lengths of 1.9055(8) Å in [Mes3SbOH][PhSO3],54 2.027(2) Å in Mes3Sb(OH)2,59 and 1.934(2) Å in (Ph3SbO)2.37 The next shortest Sb···O distance in the crystal structure of Mes3SbO is 6.980(2) Å, confirming that there are no interactions between the stiboryl groups of adjacent monomers.

Figure 1.

Figure 1

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Thermal ellipsoid plot (50% probability) of Mes3SbO. Color code: Sb teal, O red, and C black. Hydrogen atoms have been omitted for clarity.

Analysis of the previously reported crystal structures of Dipp3SbO reveals some notable deviations from local idealized geometry that were not explicitly discussed in the original report.56 Structures of two polymorphs (monoclinic and orthorhombic) of Dipp3SbO were solved and refined, and values for both polymorphs are collected in Table S3, but for brevity, the discussion here will be restricted to the monoclinic structure. Although the compound exhibits a generally tetrahedral structure, the sum of the Cipso–Sb–Cipso bond angles is 338.73(3)°, which is statistically significantly different from the value of 328.5° that would be expected for a perfectly tetrahedral molecule. The increase in the sum of these angles is consistent with the restricted ability of the bulky Dipp groups to approach each other. It is also notable that the Sb atom is not coplanar with each of the Dipp aromatic rings. This deviation can be captured with the Sb–Cipso–Cpara bond angles (expected value = 180°), which average to 169.46(2)° with a minimum value of 164.38(3)°. The analogous values for the orthorhombic polymorph are comparable and are collected in Table S3. In contrast, Mes3SbO more closely approaches an ideal tetrahedral geometry at the Sb center, with Cipso–Sb–Cipso bond angles summing to 333.1(2)°. Moreover, the Sb-Mes groups are closer to planar with an average Sb–Cipso–Cpara bond angle of 173.06(7)° and a maximum deviating value of 169.42(13)°. These results directly reflect the influence of the decreased steric bulk of the mesityl groups on the molecular structure of the monomeric stibine oxide. To confirm that these deviations do not arise from intermolecular packing forces within the crystals, we also compared the calculated gas-phase geometries (PBE0/def2-TZVPP) of Mes3SbO and Dipp3SbO (Table S3). Dipp3SbO is more distorted from an ideal VSEPR geometry than Mes3SbO, and the extent of deviation is even greater than that observed in the crystallographic data. The sum of the Cipso–Sb–Cipso bond angles is 332.01° in Mes3SbO and 339.62° in Dipp3SbO. The average Sb–Cipso–Cpara bond angles are 174.26° in Mes3SbO and 169.17° in Dipp3SbO (PBE0/def2-TZVPP). This decreased steric bulk in Mes3SbO, which is reflected in the aforementioned structural parameters, is expected to significantly impact the reactivity at the stiboryl unit.

To gain deeper insight into the electronic structure of the novel monomeric stibine oxide, we subjected Mes3SbO to a number of theoretical analyses; computational data for Mes3PnO and Dipp3PnO (Pn = P, As, Sb) are collected in Table S4. Topological analysis (PBE0-DKH/old-DKH-TZVPP) of the electron density (ρ) found a (3,–1) critical point (bcp) along the Sb–O interatomic vector, confirming the presence of a bond (Figure 2A). The value of ρ at the Sb–O bcp of Mes3SbO is almost identical to that of Dipp3SbO. As with Dipp3PnO (Pn = P, As, Sb), comparing Mes3PnO (Pn = P, As, Sb) in the Pn–O bonding region finds the lowest value of ρ for the stibine oxide. Assessment of the Laplacian of ρ (∇2ρ) reveals a minimum that is the lowest in magnitude for the stibine oxide, consistent with the weakest bond and the lowest concentration of charge in the bonding region of the series (Figure 2A). The low ellipticity (ε) of ρ along the pnictoryl Pn–O bond relative to other types of Pn–O bonds results from the cylindrically symmetrical backdonation from O-centered lone pairs to Pn–C σ* orbitals.32 As with Dipp3SbO, the HOMO and HOMO–1 of Mes3SbO feature strong contributions from the two O-centered p orbitals and are significantly destabilized relative to the corresponding orbitals for its lighter congeners (Figure 2B). We have previously demonstrated that, for the Dipp3PnO series, this variation in electronic structure manifests as comparatively enhanced basicity for the stibine oxide.57 The LUMO of Mes3SbO, although highly delocalized across the stiboryl unit and the aryl groups, nonetheless exhibits significant Sb–O σ* character with a pronounced lobe opposite the Sb–O bond (Figure 2C). The low-lying LUMO of Mes3SbO relative to its lighter congeners suggests that there will be enhanced Lewis acidity at the Sb center, as well as enhanced biphilic reactivity across the polar unsaturated stiboryl group. Natural population analysis is consistent with a highly polarized stiboryl bond with significant positive charge on the Sb atom (+2.22) and significant negative charge on the O atom (−1.23).

Figure 2.

Figure 2

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(a) Values of ρ (e–Å–3), ∇2ρ (e–Å–5), and ε for Mes3PnO (Pn = P, As, Sb) along the Pn–O bond paths, with Pn at 0.00 and O at 1.00 along the horizontal axis. The bond length is normalized to 1.00. The location of the (3, –1) critical point is shown with a dashed vertical line. Canonical molecular orbital diagrams of (B) HOMO and (C) LUMO of Mes3SbO (isovalue = 0.02). Surface plots (isovalue = 0.05) depicting (D) an O-centered lone pair NLMO, (E) the Sb–O σ* antibonding NLMO, (F) the Sb–O σ bonding NLMO, and (G) overlap of an O-centered lone pair NLMO and an Sb–C σ* antibonding NLMO. Color code: Sb teal, O red, C gray, and H white.

The relatively high delocalization of these canonical MOs (additional perspectives provided in Figures S49 and S50) prevents facile interpretation by visual inspection, but further detailed information could be obtained by analysis of the natural bond orbitals (NBOs) and natural localized molecular orbitals (NLMOs). An approximately cylindrically symmetrical Sb–O bonding NLMO is polarized toward the O atom with a 26% contribution from an Sb-centered orbital (s: 41%, p: 59%) and a 74% contribution from an O-centered orbital (s: 19%, p: 80%) (Figure 2F). The corresponding Sb–O σ* NLMO closely resembles the LUMO; we expect that increased steric access to the lobe of this orbital, compared to Dipp3SbO, will unlock enhanced Lewis acidity for Mes3SbO (Figure 2E). Two O-centered p-hybridized NLMOs recapitulate the two nearly degenerate highest occupied canonical MOs (Figure 2D). The two corresponding parent lone pair NBOs are each populated with 1.89 electrons, suggesting appreciable non-Lewis character in the electronic structure. Second-order perturbation theory analysis finds significant delocalizations of electron density from these O-centered lone pairs into Sb–C σ* orbitals (Figure 2G). Deletion calculations suggest that these back-bonding interactions are of similar strength to those present in Dipp3SbO and much weaker than those present in arsine and phosphine oxides (Table S4). Our experimental and theoretical data suggest that electronic variations between the stiboryl groups of Mes3SbO and Dipp3SbO are quite subtle, and differences in the reactivity of these two species are more likely to result from the distinctly different steric environments imparted by the aryl substituents.

Reactivity between Organotetrel(IV) Chlorides and Mes3SbO

With the more sterically accessible stibine oxide in hand, we reinvestigated PbMe3Cl as a Lewis acidic substrate. Treatment of a suspension of PbMe3Cl with Mes3SbO in DCM resulted in the rapid consumption of the solid (Scheme 2A). Addition of pentane to the mixture led to the growth of colorless crystals. 207Pb NMR analysis of the product found a single resonance at 207.6 ppm, significantly shielded relative to PbMe3Cl (413.2 ppm), consistent with the expansion of the Pb coordination sphere.62 The 1H NMR spectrum was consistent with the formation of Mes3SbO → PbMe3Cl; the equimolar Sb-bound mesityl substituents and Pb-bound methyl substituents each exhibited significantly shifted proton resonances relative to the corresponding precursors. The ortho-methyl mesityl protons remained symmetrized by rapid rotation about the Sb–Cipso bond. The Pb-bound methyl protons feature a strengthened 2JPbH coupling constant of 85.1 Hz relative to that of PbMe3Cl (65.5 Hz). The increase in 2JPbH would be consistent with the enrichment expected in the s-character of the Pb-based orbitals used to make the equatorial Pb–C bonds if the Pb atom moves to a trigonal bipyramidal geometry with three equatorial methyl substituents and axial chloride and Mes3SbO ligands. This geometry was subsequently confirmed crystallographically.

Scheme 2. Synthesis of (A) Mes3SbO → PbMe3Cl and Mes3SbO → SnMe3Cl, (B) Mes3Sb(OGeMe3)Cl and Mes3Sb(OSiMe3)Cl, and (C) Mes3Sb(OCPh3)Cl.

Scheme 2

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Mes3SbO → PbMe3Cl crystallized as the DCM solvate in space group P21/c (Figure 3A). The Sb atom maintains a tetrahedral coordination sphere and, as predicted spectroscopically, the Pb atom features a trigonal bipyramidal geometry (τ = 0.958(4)). Mes3SbO coordinates the Pb atom through the stiboryl oxygen with an O–Pb distance of 2.351(3) Å. Coordination to the Lewis acid lengthens the Sb–O bond to 1.860(3) Å, consistent with other Lewis adducts of stibine oxides and with the notion that donation of O-based lone-pair electron density to the Pb center reduces the strength of back-bonding and weakens the Sb–O bond.

Figure 3.

Figure 3

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Thermal ellipsoid plots (50% probability) of (A) Mes3SbO → PbMe3Cl and (B) Mes3SbO → SnMe3Cl. Color code: Sb teal, Pb purple, Sn blue, Cl dark-green, O red, and C black. Hydrogen atoms and solvent molecules have been omitted for the sake of clarity.

Treatment of Mes3SbO with SnMe3Cl afforded similar results (Scheme 2A). The 119Sn nucleus in the product resonates at −36.38 ppm in CDCl3, significantly upfield of Me3SnCl (169.1 ppm), and the 2JSnH coupling (69.0 Hz) is stronger in the product.63,64 These NMR data are consistent with the formation of a 5-coordinate Sn center in solution. Crystals of Mes3SbO → SnMe3Cl·(C7H8) were grown from a concentrated toluene solution, and the solid-state structure was determined by SC-XRD. Mes3SbO → SnMe3Cl·(C7H8) crystallized in space group P21/n (Figure 3B). As with the Pb-containing analogue, the tetrahedral geometry is maintained about the Sb center, and the stiboryl O atom coordinates to the Sn atom, affording a trigonal bipyramidal geometry about the Sn atom (τ = 0.839(2)). The O–Sn interatomic distance is 2.180(2) Å, and the Sb–O bond length is 1.865(2) Å.

We expanded our investigation of Mes3SbO toward reactivity with lighter organotetrel(IV) chlorides. A toluene solution of Mes3SbO was treated with GeMe3Cl (Scheme 2B). Evaporation of the solvent resulted in the growth of colorless crystals. These crystals were determined to be the toluene hemisolvate of Mes3Sb(OGeMe3)Cl by using SC-XRD (Figure 4A). In contrast to the Pb- and Sn-containing analogues, the Sb atom expands its coordination sphere to a trigonal bipyramid (τ = 0.917(2)) to incorporate the chloride ion in the axial position opposite the trimethylgermyloxide ligand. Mes3Sb(OGeMe3)Cl can be viewed as the formal addition product of GeMe3Cl across the unsaturated stiboryl bond. This addition demonstrates the capability of Mes3SbO to engage substrates in a biphilic manner with both the Lewis basic O atom and the Lewis acidic Sb atom. The Sb–O bond distance of 1.970(2) Å is significantly longer than that of the Lewis-acid-coordinated monomeric stibine oxides described above. The longer Sb–O bond length in the stiborane is expected due to the even stronger interaction between the O and Ge atoms and because of the 3-center-4-electron bonding along the Cl–Sb–O axis (which can alternatively be described as donation from the axial chloride substituent into the Sb–O σ* orbital).

Figure 4.

Figure 4

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Thermal ellipsoid plots (50% probability) of (A) Mes3Sb(OGeMe3)Cl, (B) Mes3Sb(OSiMe3)Cl, and (C) Mes3Sb(OCPh3)Cl. Color code: Sb teal, Ge light-green, Si yellow, Cl dark-green, O red, and C black. Hydrogen atoms and solvent molecules are omitted for clarity.

PXRD and elemental analysis confirm that bulk samples of the Ge-containing product match the identity of the crystal analyzed by SC-XRD. NMR analyses of this compound are complicated by apparent isomerization in solution. In toluene-d8, the 1H and 13C NMR spectra each reveal the presence of two sets of sharp signals in approximately an 8:1 ratio. Each set of signals in the 1H and 13C NMR spectra in toluene-d8 shows a single mesityl environment desymmetrized by a restricted rotation about the Sb–Cipso bond axis. The restricted rotation in each species is consistent with an increase in steric congestion at the stiboryl group caused by the trimethylgermanium motif. Initially, we suspected that one species was the addition product, as observed in the solid state, and that the other species was a Lewis adduct of the type observed for the Pb- and Sn-containing products. We note, however, that attempts to computationally optimize (BP86/def2-SVP) the geometry of either the 5-coordinate Lewis adduct Mes3SbO → GeMe3Cl or the 6-coordinate Lewis adduct (Mes3SbO)2 → GeMe3Cl only resulted in dissociation to form 4-coordinate Ge species (Figure S48). These calculations suggest that although Ge can readily achieve high coordination numbers with smaller ligands, the sterically demanding Mes3SbO ligand favors a lesser degree of substitution. The two species present in solution could alternatively be Mes3Sb(OGeMe3)Cl and [Mes3Sb(OGeMe3)]Cl. The latter is simply the trimesitylgermyloxystibonium chloride salt that would form if the chloride dissociated from the neutral trimesitylgermyloxychlorostiborane. To date, our attempts to observe the exchange between these species by VT-NMR and EXSY experiments have been unsuccessful.

Treatment of Mes3SbO with SiMe3Cl in DCM resulted in the formation of Mes3Sb(OSiMe3)Cl (Scheme 2B). The 1H and 13C NMR spectra in CDCl3 reveal a single species in solution with a single mesityl environment. The breadth of the ortho-methyl and aryl proton resonances is indicative of restricted rotation about the Sb–Cipso bond axis that is consistent with the more sterically encumbered addition product relative to Mes3SbO. Crystals of Mes3Sb(OSiMe3)Cl were grown from a mixture of DCM/pentane and analyzed by SC-XRD. Similar to the Ge-containing analogue, the solid-state structure features a 5-coordinate stiborane (Figure 4B) with an analogously lengthened Sb–O bond. Interestingly, if Mes3SbO is treated with two equivalents of SiMe3Cl in CDCl3 and heated to 50 °C, the initially formed Mes3Sb(OSiMe3)Cl converts to the deoxygenation product, Mes3SbCl2, with the generation of (Me3Si)2O (Figure S24).

The series was completed with trityl chloride. Upon combination of DCM solutions of Mes3SbO and CPh3Cl, the reaction mixture quickly became cloudy before ultimately precipitating large colorless crystals (Scheme 2C). SC-XRD analysis determined the identity of the product to be Mes3Sb(OCPh3)Cl·(CH2Cl2)1.5 (Figure 4C). Mes3Sb(OCPh3)Cl·(CH2Cl2)1.5 crystallizes in space group RInline graphic on a crystallographic 3-fold rotation axis. The steric bulk of the triphenylmethoxide ligand could favor a linear Sb–O–C bond angle, as compared to the bent Sb–O–E angles observed for Mes3Sb(OGeMe3)Cl and Mes3Sb(OSiMe3)Cl, but we anticipate that the potential energy surface is rather shallow along the Sb–O–E internal coordinate.

Activation of C–F and Si–F Bonds

The capacity of Mes3SbO to access biphilic reactivity and add substrates across the polar, unsaturated stiboryl group encouraged us to pursue more challenging bond activations. Main-group compounds with sufficiently potent Lewis acidity can abstract fluoride from C(sp3)–F bonds, and so we next investigated whether Mes3SbO could engage in C–F activation through a biphilic mechanism. Mes3SbO and C(p-MeOPh)3F were combined in a Teflon vessel and dissolved in DCM. After 5 days, a crystalline precipitate had deposited beneath an intense yellow-orange supernatant. The yellow-tinged precipitate was collected, washed, and recrystallized to yield a colorless powder. The 19F NMR spectrum of the product revealed a single signal at −89.47 ppm, indicating the presence of an 19F nucleus that is significantly deshielded relative to either Mes3SbF2 (−100.72 ppm, CDCl3) or Mes3SbF(O3SCF3) (−144.66 ppm, CD2Cl2).65 This relative deshielding is consistent with the formation of a stiborane that bears a fluoride trans to a strongly donating alkoxide ligand. The 13C NMR spectrum reveals JCF coupling with mesityl C atoms and one of the two magnetically distinct ortho-methyl C atoms of the mesityl substituents but not with any of the triarylmethyl C atoms, providing unambiguous evidence for fluoride migration. The desymmetrization of the mesityl resonances in the 1H and 13C NMR spectra provides further evidence for the existence of an asymmetrically substituted 5-coordinate stiborane in solution (Scheme 3A). SC-XRD of crystals of the C–F activation product obtained from a mixture of DCM and pentane afforded a structure of Mes3Sb(OC(p-MeOPh)3)F·(CH2Cl2)2 in space group PInline graphic (Figure 5A). As expected from the spectral data, the C–F bond of C(p-MeOPh)3F had been cleaved by the stiboryl group, with the formation of new O–C and Sb–F bonds, with interatomic distances of 1.428(2) and 2.0238(10) Å, respectively. Attempts to observe an analogous reaction with CPh3F were unsuccessful, even after extended periods of heating (Figures S38 and S39), indicating that the electron-donating OMe groups were important in the reaction described above.

Scheme 3. Synthesis of (A) Mes3Sb(OC(p-MeOPh)3)F and (B) Mes3Sb(OSiEt3)F.

Scheme 3

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Figure 5.

Figure 5

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C Thermal ellipsoid plots (50% probability) of (A) Mes3Sb(OC(p-MeOPh)3)F and (B) Mes3Sb(OSiEt3)F. Color code: Sb teal, Si yellow, F green, O red, and C black. Hydrogen atoms and solvent molecules are omitted for clarity.

This collection of promising results finally led us to test whether a stibine oxide would be able to activate a Si–F bond, such as that in a SiR3F fluorosilane. Although competing with silylium cations for binding to fluoride requires extremely high Lewis acidity, the activation of fluorosilanes by more modest Lewis acids has previously been facilitated by Lewis basic solvents that bind to and stabilize the resulting silylium cation.15,16 We hypothesized that the biphilic reactivity of the stiboryl group would be sufficiently potent to cleave the Si–F bond of a fluorosilane and generate the product of 1,2-addition across the unsaturated Sb–O bond. Addition of SiEt3F to a solution of Mes3SbO in toluene followed immediately by concentration of the reaction mixture resulted in the growth of colorless crystals. 19F NMR analysis of the product in toluene-d8 revealed a single new signal at −82.34 ppm, which is relatively close to the signal observed for Mes3Sb(OC(p-MeOPh)3)F, suggesting the reaction had proceeded cleanly within minutes at room temperature (Scheme 3B). The 13C NMR spectrum of the new product also featured a JCF coupling pattern similar to that of Mes3Sb(OC(p-MeOPh)3)F, with coupling apparent to the aryl C atoms and one of the ortho-methyl C atoms of the mesityl groups. An SC-XRD experiment confirmed the identity of the product to be Mes3Sb(OSiEt3)F (Figure 5B), which crystallized in space group RInline graphic with the molecule residing on a crystallographic 3-fold rotation axis.

The activation of Si–F bonds is most commonly achieved by extremely potent Lewis acids, so we calculated the fluoride ion affinity (FIA) of Mes3SbO to interrogate its Lewis acidity.66 Mes3SbO exhibits a modest gas-phase FIA of 216 kJ/mol relative to the extreme FIA of 910 kJ/mol calculated for Et3Si+ at the same level of theory (PBE0/def2-TZVPP). Furthermore, the FIA of Mes3Sb(OSiEt3)+ is 584 kJ/mol, which is substantially lower than that of Et3Si+. It can thus be concluded that the Lewis acidity of the Sb atom in Mes3SbO alone is insufficient to abstract fluoride from Et3SiF and that the activation of the Si–F bond must be driven in large part by stabilization of the resulting silylium unit through binding to the Lewis basic stiboryl O atom.

Conclusion

In conclusion, we report the isolation and biphilic reactivity of a novel monomeric stibine oxide, Mes3SbO. Although electronically similar to Dipp3SbO, the less encumbered steric environment about the stiboryl group unlocks a greater scope of reactivity for Mes3SbO. We have characterized the ability of Mes3SbO to act as both a Lewis base and a biphilic reagent in its reactivity with organotetrel(IV) halides. Remarkably, the biphilicity of Mes3SbO is sufficiently potent to cleave C–F and Si–F bonds and to stoichiometrically form 1,2-addition products. The activation of C–F and Si–F bonds poses a formidable challenge, and the results presented herein display how unquenched reactivity at a polar, unsaturated main-group bond can be utilized in small-molecule activation. We are continuing to explore the exciting chemistry of monomeric stibine oxides; efforts to expand the reactivity of Mes3SbO toward more practical applications in catalysis and to elucidate the reactivity of other Pn–Ch bonding motifs are currently underway.

Acknowledgments

This work was supported by the NSF through CAREER award 2236365 and MRI grant 2018501, the ACS through PRF grant 66098-ND3, and the Arnold and Mabel Beckman Foundation through a Beckman Young Investigator Award to T.C.J.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.4c05394.

  • Experimental methods; NMR data; IR data; PXRD data; thermal ellipsoid plots; crystallographic tables; computed thermochemical parameters; computationally optimized Cartesian coordinates (PDF)

  • Crystallographic experimental details; refinement parameters; structure factors (CIF)

The authors declare no competing financial interest.

Supplementary Material

ja4c05394_si_001.pdf (4.1MB, pdf)

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